The measure of naturally occurring gamma radiation as a function of depth within a well borehole is the basis of one of the earliest borehole geophysical exploration system. This type system, commonly referred to as a natural gamma ray logging system, typically comprises at least one gamma ray detector housed in a downhole tool that is conveyed along the borehole.
One type of natural gamma ray logging system comprises a logging tool that is responsive to total gamma radiation emitted by the earth formation, and the tool is conveyed along the borehole by means of a wireline. This “total” natural gamma ray wireline logging system was the first type of gamma ray measurement used in borehole geophysical exploration. Since most shales are relatively rich in naturally occurring radioactive elements, these logs are used primarily to delineate shale from other formations, or used to measure the shale content of formations. This wireline logging system is used only after the borehole has been drilled.
It is often advantageous to measure total natural gamma radiation while the borehole is being drilled. This is accomplished by conveying the tool along the borehole by means of a drill string. This type of system is commonly referred to as a total natural gamma ray logging-while-drilling (LWD) system.
Yet another type of natural gamma ray logging system comprises a logging tool that measures a spectrum of gamma radiation emitted by the earth formation. The spectrum is defined a measure of intensity of radiation as a function of radiation energy. This type of logging system is commonly referred to as a spectral gamma ray logging system. Spectral gamma ray logging tools are typically conveyed along the borehole by means of a wireline. Low count rate and detector stabilization are major problems in any type of natural spectral LWD systems
Most naturally occurring gamma radiation found in earth formations is emitted by potassium (K) and elements within the decay chains of uranium (U) and thorium (Th). Energy of naturally occurring gamma radiation measurable in a borehole environment typically spans a range of about 0.1 to less than 3.0 million electron Volts (MeV). The elements K, U and Th emit gamma radiation at different characteristic energies. Components of radiation from K, U and Th contributing to the total measured gamma radiation can, therefore, be obtained by identifying these characteristic energies using spectral gamma ray logging system. Through system calibration and modeling, these components can be subsequently related to the corresponding elemental concentrations of these elements within the formation. Elemental concentrations of K, U and Th can be used to determine parameters in addition to shale content obtained from total natural gamma ray logs. These additional parameters include, but are not limited to, clay typing, lithology identification, fracture detection, and radioactive tracer placement.
As in all nuclear logging systems, statistical precision of a measurement is maximized when the count rate of the radiation detector used to obtain the measurement is maximized. Naturally occurring gamma radiation is typically much less intense than gamma radiation induced in formation materials by sources of radiation within a logging tool. It is important, therefore, to design natural gamma ray logging tools to maximized measured gamma radiation count rate.
Measured count rate can be optimized by designing tool housings (both total gamma ray and spectral gamma ray) so that gamma radiation attenuation within the housing is minimized. The lower energy region of the measured spectrum is especially important in spectral gamma ray logging systems. Wireline spectral gamma ray logging tools often employ a tool housing fabricated with material of relatively low atomic number, rather than heavier (and stronger) materials such as steel. These so called “low Z” tool cases minimize gamma ray attenuation, especially at the lower end of the energy spectrum, thereby maximizing measured count rate for a given radiation intensity and detector size. Low Z materials often do not meet structural requirements of LWD systems.
Measured count rates can further be maximized through tool detector design. Due to the relatively high energies of the characteristic K, U, and Th gamma radiation, it is advantageous for the gamma ray detector of a given type to be dimensioned as large as practically possible to react with, and thereby respond to, these radiations. Typically, larger detectors can be disposed in wireline tools with less attenuating material between the detector and the formation. LWD systems employ a relatively thick tool housing, which is typically a collar with a drilling fluid flow conduit passing through the collar.
A gamma ray detector comprising a scintillation crystal and a cooperating light sensing device, such as a photomultiplier tube, typically yields the highest spectral gamma ray detector efficiency for a given detector volume. Gamma ray detectors undergo significant temperature changes during a logging operation. The gain of a photomultiplier tube changes as the temperature and, to a lesser extent, counting rate changes. Gain changes, often referred to as gain “shifts”, adversely affect gamma ray spectral analysis. Typically, a 100 degree Centigrade (° C.) change in temperature causes 100% change in gain. Temperature variations of this order of magnitude are not uncommon in wireline or LWD logging operations. It is, therefore, necessary to compensate for detector gain changes in order to obtain accurate and precise spectral gamma ray measurements. This compensation is especially difficult to achieve in LWD systems. As an example, significant gain changes can occur over a relatively short time interval. The data rates of available LWD telemetry systems between the downhole tool and surface equipment are typically too low to effectively monitor and to correct for rapidly occurring gain shifts. Automatic downhole gain control is, therefore, highly desirable in LWD systems.
As mentioned previously, naturally occurring gamma ray spectral measurements are typically low count rate. It is, therefore, desirable to use as much of the measurable gamma ray spectrum as possible in order to maximize statistical precision. Shock and vibration effects on low count rate systems can distort spectral shape, especially at the lower energy region of the measured spectrum. This problem is especially prevalent in LWD systems, which are exposed to harsh drilling environments.